Medicine's Secret Stat

Researchers can gather all the hard-nosed evidence they want about the effectiveness of a particular drug or treatment. But there's one figure doctors don't much talk about despite its importance. It's called number needed to treat, or NNT, a new measure developed in the past 20 years that's one of the best-kept statistical secrets in medicine.

The idea of NNT is simple enough. Most clinical trials look at how much better people do on a particular medicine compared with how they would do without it or whether they should be on a different medicine. Take statins, drugs that aim to reduce bad...